This invention relates to a method of through-casing gas detection; and to apparatuses for carrying out such a method.
In the technical field of oil production there are numerous important, technical reasons for identifying the presence of gas in a formation.
It is generally considered essential to acquire a good quality density log of a well, in order to provide for reliable gas detection.
Before completion of a well it is possible to obtain accurate density logs in open-hole. This is so even when there is mudcake in the well. Under that circumstance it is possible to compensate the density log for example using one or more of the techniques disclosed in “The Dual-Spaced Density Log—Characteristics, Calibration and Compensation.”—Samworth, The Log Analyst, February 1992.
Until quite recently it was generally regarded as impossible to obtain the high-accuracy density logs following casing of the well.
However, it is possible to approximate the material of the casing to a mudcake of high density. Consequently it is possible to employ the spine/rib technique in the form specified in the above-mentioned paper by Samworth to obtain a log that is corrected for the effects of the casing.
A drawback of this technique, however, is that it cannot simultaneously correct for the irregular, cement-filled annulus encircling the steel casing following completion of the well. In addition, the dimensions of the annulus are such as to affect the accuracy of the spine/rib method.
When logging to identify gas, one usually compares the density-derived formation porosity with that derived from a neutron log. Following calibration of these measurements to read correctly in fluid-filled porosity, the presence of gas-filled porosity makes the neutron porosity log read low and the density porosity read high. When this happens the log traces deflect in opposite directions and when plotted appropriately cross over one another in a characteristic fashion. The crossover is therefore used to indicate the presence of gas.
Because the crossover depends principally on divergent log plot directions, it is possible to detect gas by comparing the character of the neutron and density logs without necessarily analysing in detail the absolute values of the logs.
However, when logging through steel casing, cavities located radially outwardly of the casing give rise to similar log plot crossovers as gas.
The mis-characterisation of cavities as gas-filled regions of the formation is a significant disadvantage of using neutron and density logs in combination to identify gas in a formation.
A further problem concerns the use of casing collars to join adjacent lengths of tubular casing together.
There are two kinds of casing joint in common use.
In flush-jointed casing a threaded, tubular spigot of reduced diameter compared with the outside diameter of the casing protrudes from one end of a length of casing. The spigot is received in a threaded socket formed in one end of an adjacent length of casing. The socket has an inside diameter slightly greater than the inside diameter of the remainder of the casing.
When the casing lengths are screwed tightly together the effect is of a continuous wall of substantially constant thickness in the vicinity of the threaded parts.
Consequently a flush joint does not significantly affect the accuracy of gas detection logs.
A casing collar, on the other hand, that is an alternative means of joining lengths of a casing together, constitutes a steel annulus encircling the exterior of the casing in the vicinity of each joint.
The very nature of the casing collars emphasises the likelihood that they will influence the accuracy of a density log.
More specifically, casing collars cause the density log to read high density values over short distances.
Aside from securing adjacent casing lengths together, casing collars on the other hand are regularly spaced along the casing. Consequently their presence can be logged using a tool called a magnetic casing collar locator, that typically is deployed in conjunction with other downhole equipment to provide an accurate, absolute depth measurement.
There exist many oil wells whose productivity has declined in recent years. Such wells probably contain secondary hydrocarbon-bearing formations, that are not readily identifiable using conventional logging techniques. Hitherto the cost of comprehensively logging such wells using a sequence of techniques has been prohibitively high.